objtool: Add --backup

Teach objtool to write backups files, such that it becomes easier to
see what objtool did to the object file.

Backup files will be ${name}.orig.

Suggested-by: Borislav Petkov <bp@alien8.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Acked-by: Borislav Petkov <bp@suse.de>
Acked-by: Josh Poimboeuf <jpoimboe@redhat.com>
Link: https://lkml.kernel.org/r/YD4obT3aoXPWl7Ax@hirez.programming.kicks-ass.net

tools/objtool/objtool.c

@@ -17,6 +17,7 @@
 #include <stdbool.h>
 #include <string.h>
 #include <stdlib.h>
+#include <unistd.h>
 #include <subcmd/exec-cmd.h>
 #include <subcmd/pager.h>
 #include <linux/kernel.h>
@@ -44,6 +45,64 @@ bool help;
 const char *objname;
 static struct objtool_file file;
 
+static bool objtool_create_backup(const char *_objname)
+{
+	int len = strlen(_objname);
+	char *buf, *base, *name = malloc(len+6);
+	int s, d, l, t;
+
+	if (!name) {
+		perror("failed backup name malloc");
+		return false;
+	}
+
+	strcpy(name, _objname);
+	strcpy(name + len, ".orig");
+
+	d = open(name, O_CREAT|O_WRONLY|O_TRUNC, 0644);
+	if (d < 0) {
+		perror("failed to create backup file");
+		return false;
+	}
+
+	s = open(_objname, O_RDONLY);
+	if (s < 0) {
+		perror("failed to open orig file");
+		return false;
+	}
+
+	buf = malloc(4096);
+	if (!buf) {
+		perror("failed backup data malloc");
+		return false;
+	}
+
+	while ((l = read(s, buf, 4096)) > 0) {
+		base = buf;
+		do {
+			t = write(d, base, l);
+			if (t < 0) {
+				perror("failed backup write");
+				return false;
+			}
+			base += t;
+			l -= t;
+		} while (l);
+	}
+
+	if (l < 0) {
+		perror("failed backup read");
+		return false;
+	}
+
+	free(name);
+	free(buf);
+	close(d);
+	close(s);
+
+	return true;
+}
+
 struct objtool_file *objtool_open_read(const char *_objname)
 {
 	if (objname) {
@@ -59,6 +118,11 @@ struct objtool_file *objtool_open_read(const char *_objname)
 	if (!file.elf)
 		return NULL;
 
+	if (backup && !objtool_create_backup(objname)) {
+		WARN("can't create backup file");
+		return NULL;
+	}
+
 	INIT_LIST_HEAD(&file.insn_list);
 	hash_init(file.insn_hash);
 	INIT_LIST_HEAD(&file.static_call_list);